Tunnel kilns, especially in the ceramic industry, are typically divided into three zones: preheating, firing and cooling zones. The firing ware passes through these zones on kiln cars or is otherwise suitably conveyed through the kiln in succession. A gas flow is provided in a direction opposite to the direction of movement of the firing ware through the kiln. The gas flow is generated by means of ventilation units in the cooling zone and suction units in the preheating zone. This gas flow ensures that the heat generated in the intermediate firing zone by burners continually warms the firing ware in the preheating zone and the cold air introduced at the end of the cooling zone continually cools down the fired ware.
The heat transfer (heat transmission) to the firing ware occurs during both preheating and cooling essentially through the two mechanisms of radiant heat transfer and forced convection by means of the flow generated in the tunnel kiln. The entire heat transfer to the firing ware in the preheating zone A, the firing zone B, which is fitted with gas burners or other types of heaters, and the cooling zone C in a typical kiln is represented in FIG. 1A and the respective share of the heat transfer by radiation or heat transmission by convection is shown in FIG. 1B. It can be recognized that the entire heat transmission is greatest in the area of the firing zone B and that the share of heat transfer by radiation is significantly preponderant. In order to increase the speed of the firing ware through the tunnel kiln, it is therefore necessary to increase the heat transmission in the preheating zone and the cooling zone and thus obtain an even heat transmission over the entire length of the tunnel kiln.
It is known to provide air recirculation devices installed separately in the preheating zone (Annual for the Brick and Tile Industry 1992, Page 92) or in the cooling zone (European Patent Application No. 0 348 603 A3), in order to increase the heat transmission on the basis of a forced convection. However, a disadvantage here is that the heat transmission is dependent on the direction of the stream from the additional injection units. Thus there are zones with high heat transmission in the immediate vicinity of the air stream of the injection unit and zones with lower heat transmission in those parts of the firing setting which are not reached by the stream. Furthermore the necessary requirement of units effecting the circulation is relatively high.
A further measure to increase the forced convection is the use of high-velocity burners with high pulse flows (Annual for the Brick and Tile Industry 1992, Page 94). As a result, however, the convection is increased particularly in the firing zone, instead of in the preheating and cooling zone.
A further method of increasing the heat transmission in the kiln is the use of impulse burners, which are known for example from waste incinerating plants (Chip R. Stewart et al., "Application of Pulse Combustion to Solid and Hazardous Waste Incineration", 1991; Brenchly, D. L., et al, Battelle Report PNL-5301/UC-95, December 1984; Brochure Cello Hi-Efficiency Burners, Atlanta 1992). Impulse burners are suitable for setting the kiln gases in the firing zone area into vibration. In a furnace with a large firing area filled with mainly liquid, powdery or gaseous firing ware, for example in a waste incinerating plant, this method is very effective. However, it is not effective in a tunnel kiln, where the firing channel is virtually filled over the entire length with firing ware which acts as a filter dampening the pressure vibrations. Furthermore in tunnel kilns, especially in the ceramic industry and unlike waste incinerating plants or cement pipe kilns, a large number of burners with relatively small capacity are in operation, coordinated in groups. Pulsation is only then effective when at least an entire burner block works in pulsed operation.
Another disadvantage in the use of pulsating burners in tunnel kilns lies also in the fact that the pulsation is not continued sufficiently into the preheating and cooling zones because of the dampening effect of the firing setting. The increase in heat transmission, therefore, does not take place where it is most needed.